Refrigerant is a chemical that fuels the cooling process of a central air conditioner. Various parts align to push the refrigerant through the system and put the chemical through phase changes that will create the cold air for your home. Refrigerant starts as a gas in the condensing unit passes through coils for a phase change and then moves through lines into your home – where the liquid refrigerant hits an expansion valve.

If the expansion valve breaks or malfunctions, problems can occur and thwart both the proper phase change and your unit's cooling efficiency. There are a few problems that can indicate you have a broken AC expansion valve, though you should always leave the formal diagnosis up to HVAC contractors.

Sudden Loss of Cooling

Has your central air conditioner suddenly stopped putting out cold air? There are a few problems that can cause a sudden loss of cooling but if you rule out the more common problems, the cause could trace back to the expansion valve.

First, check to make sure the thermostat is set correctly and has a new battery. Next, make sure the evaporator coils inside the air handler haven't frozen over, which indicates a problem with the levels of refrigerant in your system. Call in a service tech to top off your refrigerant. You also want to make sure you can hear the fan blowing in the air handler while the unit is in operation. If the fan goes out, there is no way for the system to push out the cooled air that's created over the evaporator coils.

Did all of those parts check out healthy? The expansion valve could be malfunctioning and letting in too little refrigerant into the evaporator coils at one time. This lack of refrigerant means that the coils can't properly conduct the phase change and won't become sufficiently cooled to provide you with the cold air out of your vents.

Ask a service tech to check on your expansion valve to make sure the proper amount of refrigerant is making it into the evaporator coils.

Backwashing Refrigerant

Liquid refrigerant can backwash or reverse course back towards the condensing unit for a couple of reasons. Backwashing is a major problem because the compressor at the start of the condensing cycle isn't designed to take in a liquid refrigerant and the return of the chemical in that form can blow out your compressor. You will have a costly fix on your hands and a non-functioning air conditioner until the fix is done.

Backwashing can happen due to a problem with the evaporator coils, which will cease to perform the phase change needed to push the refrigerant on through the coils. The refrigerant will then instead "backwash" into the refrigerant supply lines. But the expansion valve is supposed to stop this from happening.

The backwashing can also happen because the valve gets stuck closed and fails to let any refrigerant liquid into the evaporator. If you have a valve and a backwash, which is usually diagnosed due to frost on the refrigerant lines, you likely need a new valve.